Photoconductive rutile titanium dioxide

ABSTRACT

Disclosed is a process for the production of TiO2 having an increased photosensitivity (while maintaining continuous tone reproduction) by the heat treatment of a hydrated oxide sol obtained from hydrolyzed organic titanic esters.

United States Patent Miyatuka Sept. 24, 1974 [54] PHOTOCONDUCTIVE RUTILE TITANIUM 3,409,429 11/1968 Ekman et a1 117/221 X DIOXIDE 3,460,956 8/1969 Dahle 423/610 X 3,653,895 4/1972 Brandon [75] Inventor: Hajime Miyatuka, Kanagawa-ken, 3 93, 94 0 1972 Foss Japan 3,706,560 12/1972 Grain..... Assigneez Rank Xerox, Ltd, London England 3,709,984 1/1973 Dantro 423/610 [22] Filed: Dec. 29, 1972 FOREIGN PATENTS O11 APPLICATIONS [21] A 1 N 319 116 1,009,379 11/1965 Great Britain 96/1.5

Primary Examiner-Roland E. Martin, Jr. [52] US. Cl 96/l.5, 96/1.8, 252/501,

252/520, 117/221, 423/598, 423/610 [57] ABSTRACT [51] Int. Cl. G03q 5/08, G03q 5/04 [58] Field of Search 423/598, 610; 96/15, 1.8; D 5 Pmcess T'PF P I" 252501 520 117 I221 mg an increased photosensitivlty (whlle maintaining continuous tone reproduction) by the heat treatment [56] References Cited ograichyittiggitzdesoienrcge s01 obtained from hydrolyzed or- UNITED STATES PATENTS g 3,066,048 11 /1962 Mitchell 117/221 6 Clams 1 Drawmg F'gure PHOTOCONDUCTIVE RUTILE TITANIUM DIOXIDE BACKGROUND OF THE INVENTION It is well known in the electrophotography art to uniformly charge an electrophotographic sensitive layer with corona ions and when the sensitive layer is exposed to an imaging light, an electrostatic latent image results, which may be developed with toner. Zinc oxide is extensively used as a photosensitive material, uniformly dispersed in an insulative resin binder. Using titanium oxide instead of zinc oxide as the photoconductive powder in electrophotography has been known as well.

A photosensitive layer composed chiefly of titanium oxide has various advantages over one composed chiefly of zinc oxide. One advantage of such a photosensitive layer is soft tone, that is, it offers suitability in the continuous reproduction of the image tone. A photosensitive layer composed chiefly of zinc oxide will generally vary in contrast tone and is often inadequate in terms of continuous tone reproduction. Another advantage of a photosensitive layer composed chiefly of titanium oxide over zinc oxide is higher whiteness. At the same time,'a photosensitive layer composed chiefly of titanium oxide also has a few disadvantages. One disadvantage is very low sensitivity Le, 10 to 100 times lower than zinc oxide. Another disadvantage is that titanium oxide dispersibility in a resin binder is lower than zinc oxide. While zinc oxide used in electropho tography processes is generally produced by vapor phase oxidation, titanium is often produced by a wet processing e.g., a sulfuric acid process. However, titanium oxide obtained by wet processing is considered difficult to wet with organic solvents due to the influence of reaction residues including impurity ions.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an improved electrophotographic sensitive material useful in continuous tone reproduction.

It is another object to provide a photosensitive layer composed chiefly'of titanium dioxide which has high sensitivity.

It is a further object of this invention to provide a photosensitive binder layer which exhibits soft tone characteristics suitable for reproduction of continuous tone images.

These and other objects may be accomplished in general by the use of titanium dioxide powder resulting from a heat treatment process at high temperatures of a fluid colloidal system (hereinafter referred to as sol) composed of a hydrated oxide obtained from organic titanate compounds through hydrolysis. The hydrated oxide sol is obtained in such a manner that the organic titanic compound is hydrolyzed in an alcoholic solution of titanic ester. The oxide sol is dried and is separated from the solvent and alcohol (a reaction by product). This process produces a titanium dioxide powder, which is treated under heat at a temperature greater than 600C in an electric furnace.

Any suitable organic titanic compound may be used. Typical examples may include tetrabutyl titanate (TBT), tetraisopropyl titanate (TPT) and tetrastearyl titanate (TST). The hydrated oxide sol is obtained in Such a manner that the organic titanic compound is hydrolyzed in an alcoholic solution of titanic ester. The

organic titanic compounds may also include as typical examples titanium acylate, hydroxytitanium stearate, isopropoxytitanium stearate and titanium chelates.

The titanium dioxide powder thus produced is mixed with and dispersed in a resin binder and diluent solvent thereby obtaining a uniform dispersion. The liquid is applied over a conductive base, and subsequently dried and hardened, resulting in a photosensitive layer. Other solvents may be used as a substitute for the alcoholic solution of titanic ester. Typical examples may include nonaqueous solutions of acetone, benzene, ethyl acetate and toluene.

The titanium dioxide powder obtained from the organic titanic compound through hydrolysis has a particle size which is approximately 0.5 micron in diameter and is about 300m /g in specific surface area. Particularly, a very bulky fine powder can result from hydrolysis in the alcoholic solvent.

The crystal formation and particle size of the metal oxide sol obtained by hydrolysis may indirectly depend upon the conditions of reaction, such as the density of the material, stirring speed, temperature and the rate of adding the material substances.

The organic titanic compound also may produce sols having different properties, depending on the conditions of reaction, and yet the final product is not influenced by completion of the heat treatment. The drying of the titanium oxide sol is done under heat at to C, thereby removing the residue of the alcoholic solvent after reaction. Hydrolysis of tetrabutyl titanate (TBT) leaves butanol as a reaction residue. Butanol is very difficult to remove. To facilitate its removal, therefore, it is preferable to wash the titanate in and replace it with methanol or ethanol which is much easier to remove. Thus, the use of methanol or ethanol as the solvent should be preferable.

Water is added during hydrolysis, but it is advisable to limit the addition to a value as close to the stoichiometric volume as possible. Excessive amounts of water added to the powder sol will cause large aggregation in the dried powder which would allow the powder to sinter when treated with heat, giving rise to the possibility that the resultant titanium oxide will be extremely low in dispersibility.

Heat treatment is carried out, for example, at a fixed temperature greater than 600C in a muffle type electric furnace for two or three hours. Since titanium dioxide has a tendency to develop an oxygen ion-short structure and is considerably colored, it should preferably be exposed to the heat in an oxidative atmosphere. Many metal oxide powders become coarser when heated at high temperatures. This is also the case with titanium dioxide where sintering begins at a temperature between 800 and 900C. In particular, titanium dioxide produced by the wet process tends to be locally sintered when heated at a temperature between 800 and 900C, giving rise to the formation of large aggregated lumps. In contrast, it was found that titanium dioxide powder produced by the hydrolysis of an organic titanic compound has less tendency to sinter when exposed to heat at 900C, therefore, maintaining the form of a finely divided bulky powder. This is presumed to be partly because, unlike the titanium dioxide produced by the wet process, the concentration of ion impurities is very low which negates sintering and partly because it is originally very fine in particle size. This difference characterizes the titanium dioxide powder obtained from the organic titanic compound.

Unlike titanium dioxide powder obtained by the wet process, the dispersibility of the titanium dioxide into a resin binder and the stability therein even when treated at high temperatures is not reduced. The titanium oxide powder sol obtained through the hydrolysis of the organic metal compound is amorphous according to the results of an X-ray diffraction analysis. When treated under heat, it begins to give rise to an anatase type diffraction peak as it reaches a temperature of approximately 300C, followed by a rutile type formation at about 500C. At temperatures greater than 600C the titanium dioxide changes completely into the rutile type crystal structure, depending on how long it is heated, or after it is heated at 600C for more than 2 hours. The titanium dioxide powder thus obtained is dispersed in'a resin binder using conventional means,

such as a ball mill, sand mill or attutor.

Choice of a binder is dependent to some degree on the developer to be employed. If one particular developer is to be employed, the resin binder should not be soluble in the developer liquid. A particularly suitable resin binder is styrenated or acryl-modified alkyd resin. If a polyisocyanate compound is used to crosslink, it should preferably be or higher in hydroxyl groups value. As polyisocyanate is easily used at 20 to 60 in hydroxyl value, a condensate of 1 mol of trimethylolpropane and 3 mols of triethylenediisocyanate, or a condensate of 1 mol of hexamethylene diisocyanate or trimethylolpropane and 3 mols' of xylene diisocyanate is utilized. Ratio-wise, polyisocyanate is blended at a ratio almost equivalent to the hydroxyl value of alkyd to a ratio several to ten times the equivalent.

As epoxy ester resin, any suitable vegetable oils in the form of fatty acid ester are useable, either as they are or modified with rosin or styrene. Typical examples may include vegetable oils, soybean oil, dehydrated castor oil and linseed oil. When combined with polyisocyanate, they should preferably be 20 or higher in hydroxyl value. Secondary hydroxyl groups left in the main chain of resin molecules as a result of the opening of the epoxy ring is so low in reactivity that crosslinking reactions are slow at ambient temperatures. For this reason, a dehydrated castor oil or a fatty acid ester is more desirable. Any suitable vinyl copolymers may be used containing a primary hydroxyl group including those of a monomer selected from Group (I) described below, one to several kinds from Group (II) and one or two from Group (III).

Group (I) Monomers containing primary hydroxyl groups are generally formulated as below. Typical examples are hydroxyethyl acrylate, hydroxyethyl methacrylate, allyl alcohol, hydroxyethyl methacrylamid, hydroxyethyl acrylamid, hydroxyethyl vinylether, etc. More generally, w-hydroxyalkyl methacrylate or m-hydroxyalkyl acrylate is also applicable.

Group (ll) Styrene, methylstyrene, alkyl acrylate, alkyl methacrylate, vinyl acetate, vinyl chloride, ethylene, etc.

Group (III) Acrylic acid, methacrylic acid, crotonic acid, itaconic acid, etc. A desirable copolymer is as mentioned in the specification of the US. Pat. No. 3,481,735, for example.

As copolymers of vinyl chloride and vinyl acetate, those containing vinyl chloride at a rate of to 50 percent (by weight ratio) are preferable, which are satisfactory if they contain maleic anhydride or vinyl alcohol as minor component. This resin is relatively hard so that it often requires the addition of a plasticizer for use. The desirable mixing ratios of photoconductive powder to resin binder range from 2:l to 1:2 (by volume).

The advantage of the process of the present invention is that it provides a photosensitive layer composed of titanium dioxide which is very high in sensitivity. Particularly, it was found that the sensitivity of the photosensitive material using titanium dioxide powder obtained by heat treatment at temperatures above 800C is l0 to times higher than that of conventional TiO Heat treatment of titanium dioxide solid powder by the wet process (where titanium tetrachloride is hydrolyzed) in the same manner did not provide such a notable improvement in sensitivity as is described above. It is a phenomenon peculiar to the hydrolyzed product from an organic metal compound that a highly sensitive titanium dioxide can be obtained by heat treatment. Through this phenomenon, a highly sensitive titanium dioxide powder is obtainable, independent of the conditions for hydrolysis of the organic metal compound.

The feature of the photosensitive layer produced by the process of the present invention is a continuous tone reproduction. Generally, titanium dioxide provides an electrophotographic sensitive layer that shows soft characteristics, and it is presumed that the continuous tone reproduction featuring the sensitive layer of the present invention is attributed to such properties of titanium dioxide.

The advantage of the process of the present invention is that it can give a very sensitive layer having a smooth surface, despite the use of heat-treated titanium dioxide powder. This is believed to be due to the fact that the powder obtained has a very fine particle size with a very large specific surface area despite it having been heat-treated. And such a powder can be obtained through the hydrolysis of an organic metal compound. Titanium dioxide obtained by hydrolysis and pyrolysis of inorganic salt of titanium, such as titanium tetrachloride or titanium sulfate, begins to sinter at a high temperature such as beyond 800C having a tendency to produce large particles. This lowers its dispersibility into the resin binder making it unacceptable in obtaining a sensitive layer as smoothly surfaced as can be obtained by the process of the present invention. After all, the sensitive layer of the present invention owes its excellence in continuous tone reproduction, one of the features thereof. to the characteristics of titanium dioxide. Because titanium dioxide is obtained by the hydro lysis of an organic compound, the salient advantages of the present invention are that the sensitivity is 10 to 100 times higher than that of the conventional TiO sensitive material, the powder is fine and highly dispersible, and a smoothly surfaced sensitive layer can be obtained. Although it is not completely understood why the titanium dioxide powder obtained from an organic compound improves the sensitive layer in sensitivity, one theory is believed to be that the powder is so fine in particle size that its crystallization under heat is progressed more effectively. By extending the process of the present invention as far as the use of titanium di oxide mixed with other photoconductive powders, such as zinc oxide or cadmium sulfide and mixtures thereof, it is possible to further improve the sensitive layer in charge retention and in sensitivity as well. Also, by the DESCRIPTION OF THE PREFERRED EMBODIMENT The following preferred examples further define and addition of some dyes (for sensitization), the process of 5 describe preferred materials, methods and techniques the present invention permits the sensitivity to be expanded to the visible region. As for dyes effectively used for sensitization of titanium dioxide, typical examples may include Auramine, Primoflavine, Fluorescein, Rose Bengal, Thioflavine, etc.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is further expalined in the following description with reference to the accompanying drawing wherein:

The FIGURE shows the characteristic curves of the electrophotographic sensitive materials (A,B,C,- D,E&F) and of a conventional electrophotographic sensitive material (G).

The light discharge curves of the FIGURE are obtained by measuring dark decrement from a corona discharge and the light difference in brightness from a tungsten filament bulb. From the light decrement curves the percentage of residual potential is determined by the following equation:

(V /V )/V /V X 100 percent where V is the potential after designated t seconds of exposure in a certain degree of brightness I, V is the potential before the exposure, V is the initial potential in the measurement of darkness decrement and V is the potential after t seconds of darkness decrement. Each calculated value and a value corresponding to log I X t are plotted to obtain a characteristic curve. The FIGURE shows the characteristic curves (A,B,C,- D,E&F) of the photosensitive layers of the respective embodiments of the invention obtained in the manner explained above. For comparisons sake, curve (G) of a conventional electrophotographic sensitive material utilizing titanium oxide obtained by a 2 hours long heat-treatment at 900C of dioxide sol resultant from the hydrolysis of titanium tetrachloride is included.

These curves suggest that the photosensitive layers of the said embodiments of the present invention are unexpectedly and markedly higher in sensitivity than the sensitive layer obtained by the conventional process. The gradients of the curve are small, indicating a soft characteristic. It is to be noted that heat-treatment at higher temperature provides higher sensitivity. Charging characteristics are as shown in the table below. The retention of the surface charge is higher as the heattreating temperature increases in a range from 600 to 800C, but shows a decrease with the temperature rising further from 800 to 900C.

Charging Characteristics of the present invention. In the examples, all parts and percentages are by weight unless otherwise specified.

EXAMPLE I 500 parts of tetrabutyl titanate (hereinafter called TBT) is dissolved in 1000 parts of methanol. To the solution being stirred vigorously is slowly added dropwise 500 parts of methanol mixed with approximately 30 parts of water. This gives rise to the hydrolysis of TBT, resulting in white sol particles. The dispersing solvent which results from hydrolysis is separated from the sol. Methanol is added to the sol particles which exist in a cakey form, and upon repeated dispersion and separation of the latter, the water and butanol left as reaction residues are removed. The sol particles in the cakey form thus obtained finally are placed in a drying oven and are dried therein at a temperature of approxi- Heat-treated titanium dioxide 900 weight parts Styrenated alkyd resin (hydroxyl value: Approx. 50) 60 weight parts Polyisocyanate resin (a condensate of trimethylol propane and trilene diisocyanate at a ratio of 1:3 in

mol) 40 weight parts To the above composition is added n-butyl acetate as a diluent, which is mixed in a ball mill for approximately 10 hours for dispersion. The resulting dispersed liquid is applied over a polyethylene terephthalate (PET) film having an aluminum-evaporated layer on the surface thereof, thereby producing an electrophotographic sensitive layer. To accelerate the hardening reaction of the resin, the PET film is rested for 24 hours at 40C. The photosensitive material thus obtained is further rested in a dark place for 48 hours or longer for adjustment to the darkness, after which it is electrically charged by positive or negative corona discharge, and is measured for darkness decrement characteristic first. Then, other parts of the charged specimen are cut out and are exposed in turn to the difference in light brightness from tungsten filament bulbs, whereby the light decrement curves are obtained.

EXAMPLE II The process according to Example I is repeated, with the exception that the heat-treatment is conducted at 800C corresponding to characteristic curve (B).

EXAMPLE III The process according to Example I is repeated with the exception that the heat-treatment is conducted at 900C corresponding to characteristic curve (C).

EXAMPLE [V The process according to Example 1 is repeated with the exception that tetraisopropyl titanate is substituted for TBT. The titanium dioxide powder obtained is high in dispersibility, giving a smoothly surfaced photosensitive layer corresponding to curve (D) the FIGURE.

EXAMPLE V Titanium dioxide (heat-treated at 600C) is produced according to Example I and mixed with zinc oxide. The mixing ratio is approximately 50:50. Characteristic curve (E) shows that the photosensitive layer thus obtained has higher contrast properties than Ex ample III with a little higher sensitivity (approximately 1.5 times higher). The charging characteristic was such that V was l90 volt and V60/VO was 85 percent under negative charge.

EXAMPLE VI Although the present examples are specific in terms of conditions and materials used, any of the above listed typical materials may be substituted when suitable in the above examples with similar results. In addition to the steps used to prepare the TiO of the present invention, other steps or modifications may be used, if desirable.

Anyone skilled in the art will have other modifications occur to him based on the teachings of the present invention. These modifications are intended to be encompassed within The scope of this invention.

What is claimed is:

l. A method for preparing a photoconductive rutile titanium dioxide powder having a high sensitivity while maintaining continuous tone reproduction comprising the following steps:

a. hydrolyzing, in a non-aqueous solvent, a titanic ester to produce an oxide so] of titanium,

b. drying said oxide so], and

c. heattreating said oxide sol at a temperature greater than 600C. for a sufficient period of time to produce said rutile titanium dioxide.

2. A composition of matter comprising titanium dioxide produced according to claim 1 and an insulative resin binder.

3. The composition according to claim 2 wherein titanium dioxide is mixed with a photoconductive material selected from the group consisting of zinc oxide, cadmium sulfide and mixtures thereof.

4. The composition according to claim 2 wherein titanium dioxide is mixed with a dye.

5. The process of claim 1 wherein the oxide sol is heated in an oxidative atmosphere.

6. The process of claim 2 wherein the oxide so] is heated in an oxidative atmosphere, 

2. A composition of matter comprising titanium dioxide produced according to claim 1 and an insulative resin binder.
 3. The composition according to claim 2 wherein titanium dioxide is mixed with a photoconductive material selected from the group consistinG of zinc oxide, cadmium sulfide and mixtures thereof.
 4. The composition according to claim 2 wherein titanium dioxide is mixed with a dye.
 5. The process of claim 1 wherein the oxide sol is heated in an oxidative atmosphere.
 6. The process of claim 2 wherein the oxide sol is heated in an oxidative atmosphere. 